1. Field of the Invention
The present invention relates generally Microwave/RF components and more specifically coupled transmission line components.
2. Technical Background
A directional coupler is a four port passive device that is used to combine, split and/or direct an RF signal within an RF circuit in a desired, predictable manner. A coupler can be implemented by placing two transmission lines in relatively close proximity to each other. Directional couplers operate in accordance with the principles of superposition and constructive/destructive interference of RF waves. When splitting a signal, the RF signal directed into the input port of coupler is split into two RF signals. A first portion of the RF signal is available at the second port and a second portion of the RF signal is available at the third port. A coupler can also be used to combine two input signals to create one output signal. An essential feature of directional couplers is that they only couple the RF power flowing in one direction.
In the splitting case, the amount of RF signal power in the first and second output signals should equal the RF signal power of the input signal. However, the coupler usually has an “insertion loss” which accounts for the differences between the input signal and the output signals. The coupled output signal and the direct output signal are out of phase with respect to each other. At the isolation port, there is destructive interference of RF waves and the RF signals cancel such that there is no appreciable signal available at the fourth port. When a directional coupler is well designed, none of the power incident from the input port is available at the isolated port. In practice, the cancellation is not perfect and a residual signal may be detected. The residual signal at the isolation port is another measure of the performance of the device. Hybrid couplers are commonly used in many wireless technologies to divide a power signal into two signals. In many instances the size of the coupler is critical for both application requirements and material cost benefits.
In many applications it is desirable for the coupler to perform symmetrically so that the functionality and the performance of a symmetrical coupler should not be dependent on which end of the device is used as the input or output. As noted above, when an RF signal is directed into the first port (i.e., the input port) of a symmetrical coupler, one 3 dB signal is available at the second port (DC port) and a second 3 dB signal is available at the third port (coupled port). At the fourth port (isolation port), no appreciable signal is available. In a symmetrical coupler (as is well known in the art), the input signal can be redirected into the second port (DC port) such that the 3 dB signals are available at the first (input) port and the fourth (isolation) port. In this arrangement, the third (coupled) port functions as the isolation port.
The coupling factor is an important property of a directional coupler and is defined as the ratio of the output power of the coupled port over the input power. Hybrid couplers exhibit a coupling factor of −3 dB because they divide the incident RF signal equally between coupled port and DC port. When there is 90 degree phase difference between the coupled port path and DC port path, hybrid couplers are called 90 degree hybrid couplers. 90 degree hybrid couplers are widely employed in RF circuits such as low noise amplifiers, power amplifiers, attenuators, and mixers. However, other coupling factors are also widely used because there is often a need for power sampling functionality. For example, −5 dB, −6 dB, −10 dB, −20 dB and −30 dB are popular coupling factor values.
More formally, coupler structures can typically be described as two transmission lines of length l with an even and odd mode impedance, Z0E and Z0O, respectively. The length of the coupler may be put in terms of the dielectric constant (∈R) of the material used to implement the transmission line in accordance with the following formula:
  l  =      c          4      ⁢              f        0            ⁢                        ɛ          r                    Where c is the speed of light and f0 is the desired center frequency.The even mode impedance is the line impedance when the two coupled lines are at the same electric potential. The odd mode impedance is the line impedance when the lines have opposite electric potential. The overall system impedance Z0 of the coupled line pair is given by:Z0=(Z0eZ0o)−1/2 The coupling factor, k, is given from the even mode and odd mode impedance parameters:
  k  =                    Z                  0          ⁢          e                    -              Z                  0          ⁢          o                                    Z                  0          ⁢          e                    +              Z                  0          ⁢          o                    
To achieve a tight coupling factor, the even mode impedance must be relatively high and the odd mode impedance should be relatively low, while maintaining the proper system impedance. For example, a 3 dB coupler in a 50 ohm system could have an even mode impedance of approximately 120.7 ohms and an odd mode impedance of approximately 20.7 ohms. If the coupler is designed as a 90 degree coupler, the length of the coupled lines is chosen to be a quarter wavelength (90°) long at the coupler's operating frequency (f0) (i.e., the frequency of the RF signal being divided or combined).
One of the main challenges that RF design engineers are facing is to reduce the overall size of the device while maintaining the part performance. Various approaches have been used to reduce coupler size, but each approach has its respective drawbacks. For example, meandered line structures exhibit an even/odd mode phase velocity imbalance that limits the operational bandwidth. Moreover, the asymmetry of the line layout results in loss of performance symmetry. The inter-digital approach has been considered as a means to achieve high coupling in smaller volume, however, it does not have the desired symmetry.
What is needed is a symmetrical hybrid coupler that addresses the needs described above. In particular, a symmetrical hybrid coupler is needed that achieves high coupling in a smaller volume. A device is further needed that provides improved power handling and lower thermal resistivity in the z-direction.